aws-rds: DatabaseCluster doesn't provide option to specify instancetype for replica instances

### Description

aws-rds: L2 construct DatabaseCluster doesn't provide option to specify instancetype for replica instances and DatabaseInstance doesn't provide an option to specify cluster name. Sometime it is required to have different instance type for writer and replica to keep high availability in cluster with keep cost low. We can do this with L1 construct but is there any plan to introduce the same in L2 construct as well ?

### Use Case

We are planning to create 2 read replica along with 1 writer for each aurora cluster to handle AZ issues, so that during an AZ failure we have cluster working in 2 AZs atleast, to keep cost inline we want to keep reader on lower instance type than writer.

### Proposed Solution

Currently I don't have a solution, if option can be added in L2 construct would be great.

### Other information

_No response_

### Acknowledge

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Start with the TypeScript L2 DatabaseCluster and DatabaseInstance entry points described in the issue, and compare them with the corresponding L1 construct behavior. Confirm the desired writer/replica instance-type and cluster-name inputs, then verify the generated construct output for one writer and two readers. Done means the L2 API supports the stated high-availability and cost-control use case.

Written by the indexing model from the issue text.